Method to allocate data bits, multicarrier transmitter and receiver using the method, and related allocation message generator

ABSTRACT

In a multicarrier transmission system, a transmitter (T, T′) sends digital data packets (D) modulated on a set of carriers to a receiver (R, R′). A subset of the carriers constituting the set of carriers has frequencies (f 1,  f 2,  f 3,  f 4 ) in predetermined frequency ranges (Amateur Radio Band) with high risk for being affected by narrowbanded interference (RFI), e.g. originating from radio amateur transmission. The data bits of the digital data packets (D) that are allocated to the subset of carriers having frequencies (f 1,  f 2,  f 3,  f 4 ) within these predetermined frequency ranges (Amateur Radio Band), are allocated thereto in a redundant way. Via an allocation message (AM) communicated between the transmitter (T, T′) and the receiver (R, R′) both are aware of the redundancy in the bit allocations. The receiver (R, R′) is capable of measuring the amount of narrowbanded interference (RFI) that affects each carrier within the subset of carriers that may be affected thereby, and can re-combine data bits allocated to carriers in this subset which carry redundant data bits so that interference immunity is improved.

[0001] The present invention relates to a method to allocate data bitsto a set of carriers as described in the preamble of claim 1, amulticarrier transmitter and multicarrier receiver performing thismethod as described in the preambles of claims 10 and 11 respectively,and an allocation message generator as described in the preamble ofclaim 12.

[0002] Such a method to allocate data bits and equipment adapted toperform such a method are already known in the art, e.g. from thearticle ‘A Multicarrier E1-HDSL Transceiver System with CodedModulation’ from the authors Peter S. Chow, Naofal Al-Dhahir, John M.Cioffi and John A.C. Binghom. This article was published in the issueNo. 3, May/June 1993 of the Journal of European Transactions onTelecommunications and Related Technologies (ETT), pages 257-266.Therein, a multicarrier transceiver system is described wherein digitaldata are modulated via Discrete Multi Tone (DMT) modulation on a set ofcarriers to be transmitted from a DMT transmitter to a DMT receiver viacopper telephone lines. The block schemes of the DMT transmitter and DMTreceiver are drawn in FIG. 4 and FIG. 5 on page 261 of the cited articlerespectively. In the DMT transmitter a bit allocation means, called adata bit encoder, allocates data bits of a digital data packet, called ablock symbol, to the different carriers. The data bit encoder theretouses formula (7) on page 260 of the article. A modulation means, i.e.the inverse fast Fourier transformer of FIG. 4, then modulates the dataon the carriers where they are allocated to, to constitute themulticarrier symbols that are transmitted over the copper telephoneline. FIG. 4A illustrates a possible constellation of data bits amongstcarriers obtained by applying the known method. At the receiver's side,a fast Fourier transformer demodulates these multicarrier symbols, andthe decoder which forms part of the DMT receiver drawn in FIG. 5 of theabove mentioned article, retrieves from each carrier the exact amount ofdata bits modulated thereon and thus performs the role of bitde-allocation means. This de-allocation means obviously has to know howmany bits are modulated on each one of the carriers so that it caneasily retrieve the exact amount of data bits from each carrier. In theknown system, the bit de-allocation means gets this knowledge duringinitialisation of the transceiver system. Indeed, as is stated on page263, in lines 22-30 of the left column, the DMT transmitter and DMTreceiver negotiate with respect to bit and energy allocation duringinitialisation. As is understood from paragraph 2.2 of the article ofPeter S. Chow et al., more particularly from lines 28-34 in the rightcolumn on page 259, certain carrier frequencies may be plagued bynarrowbanded or single-frequency disturbances. In FIG. 4A such adisturbance is represented by RFI. Forward error correction techniques,well-known in the art, can reduce the effect of such disturbances.Nevertheless, unrecoverable errors may still appear at the receiver'sside. Thereto, Peter S. Chow et al. propose in their article thebitswapping solution: bit- and energy allocations are updated so thatdata bits are no longer transmitted via affected carriers. Such are-allocation of data bits requires an additional communication betweenthe DMT transmitter and DMT receiver, similar to the communicationperformed during initialisation, since both have to get aware of the newbit-allocations. Such a communication may be time-consuming and databits may already be lost before the bits are swapped to less noisycarriers. Bitswapping thus may imply unrecoverable loss of informationif it is seen as a solution for narrowbanded interference.

[0003] A problem similar to the just described one is known from thearticle ‘Overlapped Discrete Multitone Modulation for High Speed CopperWire Communications’ from the authors Stuart D. Sandberg and Michael A.Tzonnes, an article which is published in the ‘IEEE Journal on SelectedAreas in Communications’, Vol. 73, No. 9, December 1995 on pages1571-1585 thereof. This article also describes a Discrete Multi Tone(DMT) modulation system which differs from the system described by PeterS. Chow et al. in that wavelet modulation and demodulation techniquesare used instead of Fourier transforms. The wavelet transformation is,similar to the Fourier transformation, a linear transformation whichtransforms a time domain vector into a vector in another domain. Thisother domain is defined by its base functions which are complexexponentials for the Fourier transformation, and which can be morecomplex functions, implemented by means of general filter banks such asthe cosine modulated filter bank , of another wavelet transformation.More details with respect to the wavelet transformation are found in thebook ‘Numerical Recipes in C’, written by William H. Press, Soul ATeukolsky, William T. Vetterling and Brian P. Flannery and published bythe Cambridge University Press, in paragraph 13.10 on pages 591-606entitled ‘Wavelet Transforms’. As mentioned in the left column on page1583 of the article of Sandberg and Tzannes, their multicarrier systemmay be disturbed by narrowbanded interference due to the presence ofradio frequency signals. In other words, the transmission line may pickup signals broadcasted by radio amateur transmitters as a result ofwhich some carriers in the multicarrier data symbols transported by thistransmission line may be damaged. In their article, Sandberg and Tzannesprove that their system, thanks to the wavelet modulation anddemodulation techniques, has an improved immunity for such narrowbandedinterference compared to multicarrier systems using Fourier transformmodulation and demodulation methods, due to the intrinsic betterspectral containment of the carrier waveforms. Nevertheless, also in thesystem of Sandberg and Tzannes, unrecoverable errors still have to besolved by re-allocation of data bits.

[0004] An object of the present invention is to provide a method forallocating data bits to a set of carriers and related equipment of theabove known type, but wherein unrecoverable loss of information due tonarrowbanded interference and time-consuming communications of the abovedescribed type are avoided.

[0005] According to the invention, this object is achieved by the methoddefined in claim 1, the multicarrier transmitter and multicarrierreceiver defined in claims 10 and 11 respectively, and the allocationmessage generator defined in claim 12.

[0006] Indeed, narrowbanded or single frequency interference such as dueto radio amateur signals may swap from one frequency to another, but, inaccordance to certain specifications, always stays within predeterminedfrequency ranges. As a result, only a limited number of carriers, thosehaving frequencies within these specified ranges, may be affected by thenarrowbanded interference. This limited number of carriers constitutes asubset of carriers which, according to the present invention, isprotected by modulating data thereon in a redundant way. The same databits may for instance be modulated two or three times on carriers whichform part of the subset. Alternatively, a linear or more complexcombination of data bits modulated on certain carriers may be modulatedon other carriers. One can even think of an implementation of thepresent invention wherein data bits of some multicarrier data symbolsare combined in a linear or more complex way, and wherein the combineddata symbols are sent on other carriers as part of other multicarrierdata symbols. The multicarrier receiver then can retrieve data bitswhich are unrecoverably affected from other carriers if one of thecarriers is damaged. No additional communication between themulticarrier transmitter and receiver is required. The multicarrierreceiver just has to be aware of the redundancy scheme used by thetransmitter. In other words, the receiver has to know which data bitsare duplicated on which carriers in the subset or how data bits werecombined and modulated on other carriers. This is told to the receiverby simple allocation messages which are generated for each carrier bythe allocation message generator according to the present invention.These messages are all transmitted during the initialisation procedure.Obviously, the data recovery improves if the level of redundancyincreases, but, as will be explained later on, an increased redundancylevel inevitably reduces the throughput capacity of the transmissionline. Thus, there is a trade-off between narrowbanded interferenceimmunity by redundant transmission of data and throughput capacity ofthe line.

[0007] An additional feature of the present invention is that, in afirst low-complexity implementation thereof, each data bit allocated toa carrier of the subset that may be affected, is in copy allocated toanother carrier of the subset, as defined by claim 2.

[0008] Thus, whenever a data bit is unrecoverably affected bynarrowbanded interference, a copy of this data bit can still be obtainedby the receiver from another carrier. This is not valid for alternativeimplementations of the present method wherein for instance onlyimportant data bits modulated on carriers in the subset and not any databit are given this level of protection.

[0009] Yet another feature of the present allocation method is that,still in this first particular implementation, the data bits allocatedto carriers in the lower half of this subset are in copy allocated tocarriers in the upper half of this subset, as defined by claim 3.

[0010] In this way, the carriers to which data bits and their copies areallocated are separated over a certain distance on a frequency scale.The probability that both the data bits and their copies will beunrecoverably affected by narrowbanded interference is reduced comparedto implementations of the present invention wherein carriers withneighbouring frequencies carry the data bits and their copies. It hashowever to be noted that this implementation, wherein the carriers arechosen, one in the lower half and one in the upper half of the frequencybands, may not always be recommended, since it assumes that the carriersin the upper- and lower half of this subset more or less have the samecapacity for carrying data bits. This capacity of a carrier is themaximum number of data bits that, given the power level at which thecarrier is transmitted and the signal to noise ratio of the carriermeasured at the receiver's side, is allowed to be allocated to thiscarrier. If the carriers in the upper half and lower half of the subsethave great differences in capacities, part of the capacity of this halfof the subset with the highest capacity for carrying data bits will notbe used. A disadvantageous consequence thereof is a decreased throughputof the transmission line.

[0011] Furthermore, a feature of the present invention is that, still inthis first implementation thereof, data bits allocated to a carrier withindex k in the subset are in copy allocated to a carrier with index T+k,T being half the number of carriers constituting the subset, as definedin claim 4.

[0012] In this way, the receiver can obtain all data bits from onesingle other carrier if a carrier is affected by narrowbandedinterference and does not hove to collect copies of the affected databits from a plurality of other carriers. A person skilled in the artwill recognise that the receiver complexity is reduced significantly inthis case. In addition, the probability that both carriers carrying thedata bits and their copies respectively, will be affected by the somenarrowbanded interference, is minimised since there are always at leastT-1 carriers in the set of carriers between these two carriers. Similarto what is explained already above, the capacity of the transmissionline may be inefficiently used if carriers associated with each other donot have capacities for carrying data bits that are more or less equal.

[0013] Moreover, an additional feature of the present invention is that,in a second implementation thereof, carriers in the subset may beassociated with each other, group by group, all carriers in one groupcarrying the same data bits. This feature is defined by claim 5.

[0014] Thus, the more carriers constitute one group and are modulatedwith the same data bits, the higher the protection against narrowbandedinterference, however, the lower the throughput capacity of thetransmission line. A person skilled in the art will understand thatcarriers constituting one group have to be selected carefully: theirfrequencies should be spread over a wide frequency range to have goodprotection, and their capacities for carrying data bits should becomparable so as to minimise throughput loss.

[0015] A third implementation of the present method is defined in claim6.

[0016] In this way, by combining data bits and transmitting thecombination of the data bits over other carriers than those where theoriginal data bits are sent over, redundant transmission can be achievedwith a higher throughput capacity than by duplicating or triplicatingthe data bits. This advantageous throughput capacity however will bepaid by a higher complexity in the receiver. The receiver has to be ableof re-generating the original data bits from the different versionsthereof that were sent.

[0017] An additional feature of the third implementation is defined inclaim 7.

[0018] Indeed, redundant information such as combinations of data bitsmodulated on certain carriers, may be transmitted as part of latertransmitted multicarrier data symbols, so that the probability of beingdamaged by interference is even more reduced.

[0019] An additional advantageous feature of the third implementation isdefined in claim 8.

[0020] Indeed, when data bits are linearly combined, the inverseoperation also is a linear combination of the received data bits as aresult of which the receiver complexity can be kept low.

[0021] Yet another feature of the present method is that the degree ofredundancy for transmission of data bits on carriers in the affectedrange may depend on quality of service requirements as defined in claim9.

[0022] In this way, data bits of services with strong bit errorprotection requirements, such as financial data transmission, orsensitive OAM (Operation And Maintenance) information likesynchronisation information, with strong bit error requirements, may besent with a high degree of redundancy so that their transmission iswell-protected. The transmission of data bits of services such as speechor video on demand on the other hand is subjected to less severe biterror requirements. Consequently, to optimally use the capacity of thetransmission line, it may be decided not to sent such data bits in aredundant way.

[0023] The above mentioned and other objects and features of theinvention will become more apparent and the invention itself will bebest understood by referring to the following description of anembodiment taken in conjunction with the accompanying drawings wherein:

[0024]FIG. 1 represents a block scheme of a transmission system whereinan implementation of the method according to the present invention isapplied;

[0025]FIG. 2 represents a functional block scheme of an embodiment ofthe multicarrier transmitter T′ according to the present invention;

[0026]FIG. 3 represents a functional block scheme of an embodiment ofthe multicarrier receiver R′ according to the present invention;

[0027]FIGS. 4A and 4B represent bit allocation schemes related to theprior art method and the method according to the present inventionrespectively; and

[0028]FIG. 5 represents a functional block scheme of an embodiment ofthe allocation message generator AMG′ according to the presentinvention.

[0029] In FIG. 1, a multicarrier transmitter T is coupled to amulticarrier receiver R via a copper telephone line TL. The transmitterT and receiver R in fact are transceivers capable of transmitting andreceiving multicarrier data symbols and both thus have a similarstructure with a transmitting part and a receiving part. To explain thetechnique of the present invention, it is sufficient to consider one-waytraffic over the telephone line TL. This is the reason why one of thetransceivers T and R is called a transmitter T whilst the other one iscalled a receiver R. Digital data are modulated on a set of carriers bytransmitter T and in addition transmitted over the telephone line TL. Iffor instance the principles of ADSL (Asymmetric Digital Subscriber Line)as described in the ANSI Standard Specification T1E1.4 are respected,the digital data are grouped in packets named DMT (Discrete Multi Tone)symbols, modulated on a set of 256 carriers with equidistant frequenciesand applied to the telephone line TL. This telephone line TL picks upradio amateur signals which interfere with the carriers that transportthe DMT symbols. The radio amateur signals can swap from one frequencyto another but, in accordance to national specifications, always staywithin a well known frequency range. This frequency range may be asingle frequency band as the one referred to by ‘Amateur Radio Band’ inFIG. 4A and FIG. 4B, or may comprise several separated frequency bonds.In FIG. 4A and FIG. 4B, only four carriers, the carriers withfrequencies f1, f2, f3 and f4, hove frequencies within the range‘Amateur Radio Band’. These carriers are probably affected by the abovementioned radio amateur signals. Transmitter T is supposed to know therange ‘Amateur Radio Bond’ and in a programmable or hardcoded waymemorises that the carriers with frequencies fl, f2, f3 and f4 may beaffected by radio interference in this range.

[0030] Transmitter T′ in FIG. 2 and receiver R′ in FIG. 3 areillustrated by a functional block scheme. Transmitter T′ and receiver R′are able to transmit and receive respectively the above mentionedmulticarrier data symbols and may replace transmitter T and receiver Rin FIG. 1 since it is supposed that T′ and R′ are able to communicatewith each other via the telephone line TL, drawn in FIG. 1.

[0031] Multicarrier transmitter T′ includes a bit allocation means BAM,a modulator MOD, a digital to analogue converter DAC, and a lineinterface LIN. This bit allocation means BAM, the modulator MOD, thedigital to analogue converter DAC and the line interface LIN are cascadeconnected between an input terminal IT and an output terminal OT oftransmitter T′. The multicarrier transmitter T′ further is equipped withan allocation message generator AMG whose output is coupled to a secondinput of the bit allocation means BAM. It has to be noted that insteadof Interconnecting IT and an output of AMG with respective inputs ofBAM, a multiplexer may be inserted to first and second inputs of whichIT and the output of AMG are coupled. An output of this multiplexer isthen coupled to an input of BAM.

[0032] Multicarrier receiver R′ in FIG. 3 also includes a fine interfaceLIN′, and further is provided with an analogue to digital converter ADC,a demodulator DEM, and a bit de-allocation means BDA which are allcascade connected between an input terminal IT′ and an output terminalOT′ of the receiver R′. In addition, the receiver R′ is equipped withinterference measuring means IMM and a diversity means DIV. An output ofthe analogue to digital converter ADC and an input of the interferencemeasuring means IMM are interconnected, an output of the interferencemeasuring means IMM is coupled to an input of the diversity means, andoutputs of the diversity means are connected to control inputs of thedemodulator DEM and of the bit de-allocation means BDA.

[0033] In the following paragraphs, the technique of the presentinvention is explained by describing in a detailed way the working ofthe different functional blocks BAM, MOD, DAC, LIN and AMG in themulticarrier transmitter T′, and LIN′, ADC, DEM, BDA, IMM and DIV in themulticarrier receiver R′. Thereto, the transmitter T′ of FIG. 2 issupposed to send multicarrier data symbols to the receiver R′ of FIG. 3via the telephone line TL of FIG. 1.

[0034] Transmitter T′, as already said, knows that the carriers withfrequencies f1, f2, f3 and f4 constitute a subset of carriers that maybe affected by narrowbanded interference. The bit allocation means BAMdivides this subset into a lower part, comprising the carriers withfrequencies f1 and f2, and an upper part, comprising the carriers withfrequencies f3 and f4. The first carriers of these parts, i.e. thecarrier with frequency f1 for the lower part and the carrier withfrequency f3 for the upper part, are associated with each other, andsimilarly, the second carriers of these parts, i.e. the carrier withfrequency f2 for the lower part and the carrier with frequency f4 forthe upper part, are associated with each other. To associated carriers,such as f1 and f3 or f2 and f4, the bit allocation means BAM allocatesidentical data bits of an incoming data bit stream D. To determine theamount of data bits that is allocated to each one of the carriers, acommunication is set up between the transmitter T′ and receiver R′during an initialisation protocol. For each carrier in the set ofcarriers whereon the multicarrier data symbols are modulated, a signalto noise ratio (SNR) measurement is executed. The measured signal tonoise ratio (SNR) values allow the bit allocation means BAM to determinethe amount of data bits that is allowed to be allocated to each carrier.In other words, the capacity for carrying data bits is measured for eachcarrier. (The capacity of a single carrier, as is known by a personskilled in the art, can be increased by power boosting the carrier, i.e.by transmitting the carrier at a higher power level.) After determiningthe capacity of each carrier, the bit allocation means BAM then canallocate to each carrier the maximum amount of bits allowed to beallocated thereto. This is for instance done by a known transmitter, notshown in any of the figures, which generates the bit constellation drawnin FIG. 4A. Alternatively, the bit allocation means BAM con allocate apredetermined number of bits, for instance the amount of bits thatconstitutes one single DMT (Discrete Multi Tone) symbol in ADSL(Asymmetric Digital Subscriber Line), and can spread these bits over thecarriers in the traditional way, i.e. optimising the individualsignal-to-noise margins of the carriers if the number of bits to beallocated is below the number of bits that can be allocated, andminimising the needed power boosts if not. From FIG. 4A it is seen thatthe 16 carriers shown therein have a capacity of 4, 3, 2, 4, 4, 2, 2, 3,1, 5, 4, 5, 1, 3, 2, and 2 bits respectively, without power boost, andthus also carry these amounts of bits in the known method. The carrierswith frequencies f1, f2, f3, and f4 can carry 2, 3, 1 and 5 bitsrespectively if they are transmitted without power boost. As alreadyexplained, according to the present invention, the bit allocation meansBAM of FIG. 2 decides to allocate identical data bits to the carrierswith frequencies f1 and f3, and also to the carriers with frequencies f2and f4. Without power boost, only one bit is allowed to be allocated tothe carriers with frequencies fl and f3 because the carrier withfrequency f3 is not capable of transporting more bits. Thus, bitallocation means BAM allocates 1 bit to the carrier with frequency f1and a copy of this bit to the carrier with frequency f3. Similarly, bitallocation means BAM allocates 3 data bits to the carriers withfrequencies f2 and f4. Although carrier f4 is able to carry data bitswithout power boost, carrier f2 is not capable to transport more than 3data bits and determines the amount of data bits that will be allocatedto both carriers. The data bits allocated to the carriers withfrequencies f2 and f4 are again copies of each other The completeconstellation for the 16 carriers obtained by applying the presentmethod, is shown in FIG. 4B. All carriers carry the same amount of bitsas they are carrying in FIG. 4A except the carriers in the range AmateurRadio Band. Therein, 4 data bits are allocated twice instead of the 11data bits which were allocated by the known method in FIG. 4A.

[0035] Referring to FIG. 2 again, the modulator MOD modulates the databits on the carriers where they are allocated to, e.g. by QAM(Quadrature Amplitude Modulation), transforms the multicarrier datasymbols from frequency domain to time domain by a modulation operation,which is in a preferred implementation, the inverse fast Fouriertransform, and optionally extends the multicarrier data symbols with acyclic prefix to compensate for intersymbol interference due to thelength of the transmission line impulse response. The digitalmulticarrier data symbols are converted into an analogue signal by thedigital to analogue converter DAC and adapted to be suitable fortransmission over the telephone line TL by the line interface LIN. Thesignal S at the output of the line interface LIN is applied to thetransmission line TL via output terminal OT.

[0036] Before transmitting the multicarrier data symbols to the receiverR′, this receiver R′ is told via allocation messages AM how many bitsare allocated to each one of the carriers. The allocation messagegenerator AMG of FIG. 2 thereto generates an allocation message AM foreach carrier. Such an allocation message AM contains an identificationof the frequency fi of the carrier where it is associated with, anamount of bits, bi, allocated to the carrier with this frequency fi, thepower level or energy level, pi, at which the carrier with frequency fiis transmitted, and a single bit, rp, which is set when the carrier withfrequency fi carries the some information as the carrier for which theimmediately preceding allocation message was transmitted. For thecomplete range of carriers, the allocation messages are transmitted inincreasing order of the amount of bits bi allocated thereto. Thus, sincethe carriers with frequencies f1 and f3 carry only one data bit, theirassociated allocation messages are transmitted first and have thefollowing contents:

[0037] Later on, the allocation messages related to the carriers withfrequencies f2 and f4 are transmitted. They have the following contents:

[0038] At the receiver's side, receiver R′ interprets the allocationmessages and understands that the carrier with frequency f3, transmittedat an energy level of 0.75 dB, carries 1 data bit which is a copy of thedata bit that is modulated on the carrier with frequency fl, transmittedat a power level of 0.69 dB. Similarly, the receiver R′ learns from thelatter two allocation messages that the carrier with frequency f4,transmitted at a power level of 0.73 dB carries 3 data bits which areidentical to the 3 data bits modulated on the carrier with frequency f2which is transmitted at a power level of 0.80 dB.

[0039] The functional blocks which form part of the allocation messagegenerator AMG of FIG. 2, are shown in FIG. 5 where the allocationmessage generator is referred to by AMG′. A carrier identificationgenerator CG, a bit amount generator BG, a power level generator PG anda redundancy information generator RG generate the information fi, bi,pi and rp respectively to be embedded in the fields of an allocationmessage AM. An embedder EM constitutes the allocation message AM andfills the respective fields with the information generated by thecarrier identification generator CG, the bit amount generator BG, thepower level generator PG and the redundancy information generator RG.Via the bit allocation means BAM where the allocation messages AM areincluded in the bitstream or directly mapped on a specific carrier, themodulation means MOD, the digital to analogue converter DAC and the lineinterface LIN in FIG. 2, the allocation messages AM are transformed intoanalogue signals suitable for transmission over the telephone line TL ofFIG. 1.

[0040] By interpreting the allocation messages, the multicarrierreceiver R′ knows that part of the data bits are transmitted twice. Thereceiver R′ now can demodulate the data bits. The line interface meansLIN′ and analogue to digital converter ADC adapt the signal S′transported over the telephone line TL of FIG. 1 and received via inputterminal IT′, and transform this signal S′ into multicarrier digitaldata symbols. The interference measurement means IMM determines for allcarriers that transport redundantly transmitted data bits (the carrierswith frequencies f1, f2, f3 and f4 in the above described example), theamount of narrowbanded interference that affects these carriers. Themeasured interference values are then applied to the diversity means DIVwhich manages the re-combination of the data bits in an optimal way. Thediversity means DIV in other words decides which bits shall be takeninto account and controls the demodulator DEM and bit de-allocationmeans BDA so that the re-combined bits appear at the output OT′ of thereceiver R′. The way wherein the data bits are re-combined can takeseveral forms. Selection diversity for instance implies that the bitswhich are transmitted on carriers with the best estimated signal tonoise ratio (SNR) are taken for demodulation. A preferred cost-effectiveimplementation of the invention should use selection diversity since itis close to optimal when the SNR differs significantly for differentversions of the data bits, and it does not require a high receivercomplexity. In FIG. 4A and FIG. 4B, it is assumed that a radio amateursignal RFI is disturbing the transmission at a frequency between f2 andf3. The effect of this interferer RFI on the carriers with frequenciesf2 and f3 will be higher than the impact on the carriers withfrequencies f4 and f1 respectively. Consequently, the interferencemeasuring means IMM measures a higher affection for the carriers withfrequencies f3 and f2 than for the carriers with frequencies f1 and f4respectively. When the measured interference values are applied to thediversity means DIV, this diversity means DIV decides to retrieve thedata bits transmitted in the affected range from the carrier withfrequency f1 instead of the carrier with frequency f3, and from thecarrier with frequency f4 instead of the carrier with frequency f2 incase of selection diversity. Alternatively, instant selection diversitymay be applied. This means that amongst different versions ofinformation symbols, the information symbol which is closest to adecision point of a constellation diagram is taken for demodulation.Another alternative diversity method is called ‘Maximal RatioCombining’. Therein, the information symbol that is taken fordemodulation is calculated as a linear combination of differentinformation symbol versions which are each weighted by a coefficientequal to the SNR of that version divided by the sum of the SNR's of allinformation symbol versions. Yet, in another alternative diversitymethod named ‘Equal Gain Combining’, the information symbol that istaken for demodulation is calculated as the average of the differentversions of that information symbol. The preferred cost-effectiveversion of the present invention, as already described above, usesselection diversity for complexity reasons.

[0041] The demodulator DEM extracts the cyclic prefix from thetransmitted multicarrier symbols, fast Fourier transforms the symbols toobtain their frequency domain representation from the time domainrepresentation and demodulates, e.g. by QAM demodulation, the data fromthe carriers. Under control of the diversity means DIV, the bitde-allocation means BDA retrieves the correct amount of bits from eachcarrier. Thus, in the affected range, ‘Amateur Radio Band’, zero bitsare retrieved from the carriers with frequencies f2 and f3, one bit isretrieved from the carrier with frequency f1, and 3 bits are retrievedfrom the carrier with frequency f4. The retrieved bits leave thereceiver R′ via output terminal OT′.

[0042] A first remark is that the above description of an embodiment ofthe present invention is limited to describing the functions performedby the different blocks drawn in the figures. From this functionaldescription, a person skilled in the art can derive how to implementeach one of the drawn blocks.

[0043] A further remark is that, although the data in the abovedescribed embodiment are transported over a telephone line TL, theapplicability of the present invention is not restricted by thetransmission medium via which the data are transported. In particular,any connection between a transmitter T and a receiver R, e.g. a cableconnection, a satellite connection, a radio link through the air, and soon, may be affected by narrowbanded interference, and thus can beprotected by the method according to the present invention if the dataare modulated on a set of carriers. The invention also is not onlyrelated to ADSL (Asymmetric Digital Subscriber Line) or similartechniques wherein DMT (Discrete Multi Tone) modulation is used. Aperson skilled in the art will be able to adapt the above describedembodiment for example so that it is applicable to systems wherein DWMT(Discrete Wavelet Multi Tone) modulation is used. Therein, the abovementioned Fourier transform and inverse Fourier transform are replacedby wavelet transformations.

[0044] It is further noticed that the origin of the narrowbandedinterference is of no importance for applicability of the presentinvention. Whether the disturbing signals are transmitted by a radioamateur, as supposed in the above described example, by a taxi, or bythe police, or are caused by metallic noise as in the above citedarticle from Peter S. Chow, or are originating from yet another sourceis not relevant. Whenever a transmitter T expects that some of thecarriers whereon data have to be modulated, may be affected bynarrowbanded interference, he can protect the data bits transmitted viathese carriers by transmitting these data bits in a redundant way.Carriers laying in the distress bands for instance also may be protectedthis way. Via the allocation messages, the receiver R is told whichcarriers are modulated with the duplicates of the data bits.

[0045] Another remark is that the modulation type also is irrelevantwith respect to applicability of the present invention. In the abovedescribed embodiment, data are modulated on and demodulated from the setof carriers via QAM modulation and demodulation respectively.Alternatively, data may be modulated by phase modulation, frequencymodulation, PSK (Phase Shift Keying) or any other modulation technique.

[0046] In the above described embodiment, any data bit transmitted inthe frequency range that may be affected is transmitted twice. As aresult, even if 50 percent of the carriers in this range are distortedby interference, all data can still be retrieved by the receiver R′.Obviously, higher or lower levels of protection are obtained bymodifying the redundancy scheme. Data bits may be transmitted 3 or 4times to the receiver, data bits may be combined in a linear or morecomplex way, and some of the data bits may be excluded for redundanttransmission since loss thereof has no large impact on the performanceof the system. The redundancy scheme, still in alternative versions ofthe present invention, may be optimised with respect to throughput ofthe transmission line. Copies of data bits that are modulated on carrierf1 not necessarily hove to be allocated to one single other carrier, butmay be distributed over more than one other carrier. In this way, thecarriers can be filled better. A disadvantage however is that in suchimplementations of the present invention, the contents of the allocationmessages and the means in the receiver R which have to interpret theseallocation messages become more complex. An improved throughput of thetransmission line can also be obtained by allocating additional databits to each carrier which can carry more bits than the duplicates ofits associated carrier. In the above described example, f1 and f4 aresuch carriers.

[0047] It is also noticed that, in yet another implementation of thepresent invention, data bits may be duplicated and modulated on carrierswith frequencies outside the range that is affected by narrowbandedinterference.

[0048] While the principles of the invention have been described abovein connection with specific apparatus, it has to be clearly understoodthat this description is made only by way of example and not as alimitation on the scope of the invention.

1. Method to allocate data bits of digital data packets (D) to a set ofcarriers in a multicarrier transmission system wherein said set ofcarriers, after being modulated with said data bits (D), is transmittedfrom a transmitter (T, T′) to a receiver (R, R′) via a transmission link(TL), a subset of said set of carriers having frequencies (f1, f2, f3,f4) within predetermined frequency ranges (Amateur Radio Band) with highprobability for being affected by narrowbanded interference (RFI)compared to carriers having frequencies outside said predeterminedfrequency ranges (Amateur Radio Band), characterised in that at leastpart of the data bits of said digital data packets (D) that areallocated to carriers of said subset of carriers, are allocated in aredundant way.
 2. Method according to claim 1, characterised in thateach data bit of said data bits that is allocated to a carrier in saidsubset of carriers, is also allocated to another, second carrier in saidsubset of carriers.
 3. Method according to claim 2, characterised inthat each said data bit that is allocated to a carrier in the lower halfof said subset, said lower half being constituted by half the number ofsaid carriers forming part of said subset having the lowest frequencies(f1, f2) therein, is also allocated to a carrier in the upper half ofsaid subset, said upper half being constituted by half the number ofsaid carriers forming part of said subset having the highest frequencies(f3, f4) therein.
 4. Method according to claim 3, characterised in that,within said subset of carriers, each data bit that is allocated to acarrier with index k is also allocated to a carrier with index T+k, Tbeing half the number of carriers forming part of said subset ofcarriers.
 5. Method according to claim 1, characterised in that eachcarrier of said subset is associated with at least one second carrier ofsaid subset, said data bits being allocated to said carrier beingidentical to said data bits allocated to each one of said at least oneassociated second carrier, and the number of data bits being allocatedto each one of said carrier and said at least one associated secondcarrier being equal to the minimum one of maximum numbers of data bitsthat is allowed to be allocated to said carrier or said at least oneassociated second carrier.
 6. Method according to claim 1, characterisedin that a combination of said data bits that are allocated to carriersin said subset of carriers, is allocated to other carriers.
 7. Methodaccording to claim 6, characterised in that said combination of databits is allocated to other carriers as part of other multicarrier datasymbols.
 8. Method according to claim 6, characterised in that saidcombination is a linear combination.
 9. Method according to claim 1,characterised in that the degree of redundancy for said data bits whichare allocated in a redundant way, is determined on the basis of qualityof service requirements.
 10. Multicarrier transmitter (T, T′) adapted totransform a sequence of digital data packets (D) into multicarrier datasymbols (S) and to apply said multicarrier data symbols (S) via anoutput terminal (OT) to a transmission link (TL) to be transmittedthereover, said multicarrier transmitter (T, T′) including between aninput terminal (IT) and said output terminal (OT) the cascade connectionof: a. bit allocation means (BAM), adapted to allocate data bits of saiddigital data packets (D) to carriers of a set of carriers whereon saiddata packets (D) have to be modulated, a subset of said set of carriershaving frequencies (f1, f2, f3, f4) within predetermined frequencyranges (Amateur Radio Band) with high probability for being affected bynarrowbanded interference (RFI) compared to carriers having frequenciesoutside said predetermined frequency ranges (Amateur Radio Band); and b.modulation means (MOD) adapted to modulate said data bits on saidcarriers where they are allocated to, to thereby constitute saidmulticarrier data symbols (S), characterised in that said bit allocationmeans (BAM) is adapted to allocate data bits in a redundant way to saidcarriers in said subset of carriers having frequencies (f1, f2, f3, f4)within said predetermined frequency ranges (Amateur Radio Band) withhigh probability for being affected by narrowbanded interference (RFI).11. Multicarrier receiver (R, R′) adapted to transform multicarrier datasymbols (S′) received from a transmission link (TL) via an inputterminal (IT′) into a sequence of digital data packets (D′), saidmulticarrier receiver (R, R′) including between said input terminal(IT′) and an output terminal (OT′) thereof the cascade connection of: a.demodulation means (DEM), adapted to demodulate said multicarrier datasymbols (S′) from a set of carriers where they are modulated on, asubset of said set of carriers having frequencies (f1, f2, f3, f4)within predetermined frequency ranges (Amateur Radio Band) with highprobability for being affected by narrowbanded interference (RFI)compared to carriers having frequencies outside these predeterminedfrequency ranges (Amateur Radio Bond); and b. bit de-allocation means(BDA), adapted to retrieve from each carrier of said set of carriers theexact number of data bits that was modulated thereon, characterised inthat said multicarrier receiver (R, R′) further includes: c.narrowbanded interference measurement means (IMM), adapted to measurefor each carrier in said subset of carriers the amount of narrowbandedinterference (RFI) by which said carrier is affected; d. diversity means(DIV), an input of which is coupled to an output of said narrowbandedinterference measurement means (IMM) and respective outputs of which arecoupled to a control input of said demodulation means (DEM) and acontrol input of said bit de-allocation means (BDA), and adapted todecide which data bits amongst redundantly allocated data bits are takenfor demodulation and re-combination; and further characterised in that:e. said demodulation means (DEM) is adapted to demodulate said data bitstaken by said diversity means (DIV); f. said bit de-allocation means(BDA) is adapted to retrieve and re-combine said data bits taken by saiddiversity means (DIV).
 12. Allocation message generator (AMG, AMG′),adapted to generate an allocation message (AM) to be communicatedbetween a multicarrier transmitter (T, T′) and a multicarrier receiver(R, R′) in a multicarrier transmission system wherein digital datapackets (D) will be transmitted between said multicarrier transmitter(T, T′) and said multicarrier receiver (R, R′) via a transmission link(TL) after being modulated on a set of carriers, a subset of carriersconstituting said set of carriers having frequencies (f1, f2, f3, f4)within frequency ranges (Amateur Radio Band) with high probability forbeing affected by narrowbanded interference (RFI) compared to carriershaving frequencies outside these frequency ranges (Amateur Radio Band),said allocation message generator (AMG, AMG′) including: a. a carrieridentifier generator (CG), adopted to generate a first parameter (fi)referring to one carrier of said set of carriers where said allocationmessage (AM) is related to; b. a bit amount generator (BG), adapted togenerate a second parameter (bi) representing an amount of data bitsthat is allocated in said multicarrier transmitter (T, T′) to saidcarrier where said allocation message (AM) is related to; c. a poweramount generator (PG), adapted to generate a third parameter (pi)representing a power level at which said carrier where said allocationmessage (AM) is related to, is transmitted; and d. embedding means (EM),respective inputs (11, 12, 13) of which are coupled to outputs of saidcarrier identifier generator (CG), said bit amount generator (BG), andsaid power amount generator (PG) respectively, said embedding means (EM)being adapted to embed said first parameter (fi), said second parameter(bi) and said third parameter (pi) in respective fields of saidallocation message (AM), characterised in that said allocation messagegenerator (AMG, AMG′) further includes: e. a redundancy parametergenerator (RG), adapted to generate a fourth parameter (rp) indicatingwhether said carrier where said allocation message (AM) is related to,carries redundant information or not; and further characterised in thatf. said embedding means (EM) is provided with an additional input (14)whereto an output of said redundancy parameter generator (RG) iscoupled, said embedding means (EM) being adapted to embed also saidfourth parameter (rp) in a respective field of said allocation message(AM).